Cutting a hole in a "Plane"
Assuming you want rectangular windows and doors:

1) Create a cube and adjust it to the required size of the window.

2) Move the cube so that it goes through the wall where you want the window to be.

3) With the cube still selected, use Ctrl-A, Rotation and Scale.

4) Select the wall object.

5) Go to the Modifiers tab (the one with the wrench on it).

6) Add a Boolean modifier.

7) In the modifer properties, change Operation to Difference.

8) Click the Object field and choose the cube.

9) Click Apply.

10) Move the cube and the wall should now have a hole in it.